Since energy issues become increasingly urgent in the world

considerable interest has focused on the utilization of low-grade waste heat.Organic Rankine cycle(ORC) is one of the effective routes to transform the low-grade waste heat to electricity.In this study

the maximum useful output work and exergy efficiency of the ORC are selected as the objective functions.10 different working fluids are chosen to comprehensively investigate and analyze the important characteristics of the two objective functions in the subcritical state.The results show that an optimal evaporating temperature exists for each working fluid

at which the useful work is the maximum.Moreover

the higher the critical temperature

the higher the optimal evaporating temperature.It is found that for the same heat source

the exergy efficiency decreases with the increase of the pinch point temperature difference.For the same pinch point temperature difference

the exergy efficiency has a maximum value when the heat source inlet temperature is twice of the pinch point temperature difference lower than the critical temperature.Otherwise

the exergy efficiency keeps increasing with the heat source temperature.These results provide some valuable guidance to optimize the ORC system and screen working fluid.